Help fixing parser for negative exponents to **

[Roger Sayle]

Unfortunately, I don't have access to the official grammars for
fortran95 and fortran2003, but I suspect that there's a bug in
gfortran's parser for processing power expressions.  Currently,
we fail to parse unary minus (and unary plus) expressions
immediately following an exponentiation/power operator.

       X = 2.0
       Y = X**-1.5D0
       PRINT 10, Y
10     FORMAT (F8.5)
       END

       X = 2.0
       Y = X**+1.5D0
       PRINT 10, Y
10     FORMAT (F8.5)
       END

       X = 2.0
       Y = 1.5
       Z = X**-Y
       PRINT 10, Z
10     FORMAT (F8.5)
       END


I believe these are all valid, and compile fine with g77.  They might
however be a "common" extension, so I thought I'd check with this list
first.  The examples in gfortran.fortran-torture/execute/power.f90 all
use parenthesis around negative exponents.

Rather than just file a bug report, I'd like some advice on how best
to fix this problem.  It looks as though the parser in matchexp.c
is based upon some official/formal description, using terms like
"level 1" expression and "level 2" expression.  If this is the case
there may just be a transcription problem encoding these functions.
Alternatively, handling negative exponents belongs either in
match_mult_operand itself, or instead of match_mult_operand directly
calling itself recursively, we need another class of operand to match.
Yet another possibility is for match_primary to handle unary plus and
minus operators.


Checking bugzilla it looks like this problem hasn't been reported
previously.  My guess is that for an experienced front-end hacker
it might be a ten minute job to correct.  If not, I can file a PR
so we can track the issue.

Many thanks.

Roger
--
Roger Sayle,                         E-mail: roger@eyesopen.com
OpenEye Scientific Software,         WWW: http://www.eyesopen.com/
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